Abstract

Thermoplastic hot-melt impregnation is widely regarded as a promising route for producing continuous fiber-reinforced composites. However, its effectiveness in real industrial processing chains remains insufficiently validated. This study investigates the performance of melt-impregnated thermoplastic tapes within a full manufacturing sequence including pultrusion and compression moulding. Glass fiber reinforced polypropylene (GF/PP) tapes produced via direct melt impregnation (Tape DMP) are compared with powder-based towpregs and commercial tapes. Results demonstrate that hot-melt impregnation significantly improves initial fiber wet-out and resin distribution, but does not achieve complete impregnation. Microscopy and mechanical testing reveal that full consolidation occurs only after subsequent processing stages. The study establishes that thermoplastic composite manufacturing is inherently a multi-stage impregnation process, where each stage contributes incrementally to final material performance. This work provides critical insights into process design, highlighting the importance of balancing viscosity, pressure, and time across different manufacturing stages.

Keywords

Thermoplastic composites; Melt impregnation; Pultrusion; Compression moulding; UD tape; Towpreg; Fiber wet-out; Multi-stage consolidation

1. Introduction

Thermoplastic composites are increasingly adopted in structural applications due to their recyclability, impact resistance, and rapid processing potential. Despite these advantages, achieving consistent and high-quality impregnation of continuous fibers remains a key challenge.

In contrast to thermoset systems, thermoplastics exhibit high melt viscosity, which restricts resin penetration into dense fiber bundles. As a result, the assumption that impregnation can be completed in a single processing step is fundamentally flawed.

Recent developments in melt-impregnated tape production have improved initial wetting and material uniformity. However, the actual effectiveness of such tapes in downstream manufacturing processes has not been fully validated. This study addresses that gap by analyzing the complete process chain from hot-melt impregnation to final consolidation.

2. Literature Review

Existing research on thermoplastic composites has primarily focused on improving individual stages of production, including prepreg manufacturing, pultrusion, and compression moulding. Melt impregnation methods have been shown to enhance fiber wetting compared to powder-based systems, while pultrusion offers efficient continuous shaping.

However, previous studies often treat impregnation as a single-stage process, overlooking the cumulative effects of multiple processing steps. Limited work has been conducted on understanding how impregnation evolves throughout the manufacturing chain.

Recent experimental observations suggest that even well-impregnated tapes may still exhibit incomplete wetting and void formation after pultrusion. This indicates that consolidation is not solely dependent on the prepreg stage, but rather on the interaction between multiple processing conditions.

3. Methodology

3.1 Materials

Glass fiber rovings (2400 tex) were used as reinforcement, combined with polypropylene (PP) as the thermoplastic matrix. Three types of intermediate materials were evaluated:

• Powder-based towpreg (PCT)
• Commercial melt-impregnated tape
• In-house melt-impregnated tape (Tape DMP)

3.2 Prepreg Production

Tape DMP was produced using a direct melt impregnation system incorporating:

• Fiber spreading
• Molten polymer bath
• Impregnation pins
• Heated die shaping

The process operated at approximately 180°C with a line speed of 4 m/min and a target polymer content of 30 wt%.

3.3 Processing Chain

The materials were processed through two sequential stages:

1. Pultrusion — continuous shaping under heat and moderate pressure
2. Compression moulding — final consolidation under higher pressure and extended time

3.4 Characterization

Mechanical properties were evaluated through flexural and tensile testing. Microstructural analysis was performed using optical microscopy to assess fiber distribution, void content, and impregnation quality.

4. Results

4.1 Pultrusion Stage

Pultruded profiles produced from melt-impregnated tapes exhibited improved stiffness due to higher fiber volume fractions. However, microscopy revealed:

• Non-uniform fiber distribution
• Presence of voids
• Incomplete polymer penetration within fiber bundles

These findings confirm that pultrusion alone does not achieve full impregnation.

4.2 Compression Moulding Stage

After compression moulding, significant improvements were observed:

• Reduction in void content
• More uniform fiber distribution
• Enhanced interfacial bonding

Mechanical properties increased substantially, with flexural strength improvements exceeding 70% in some cases.

4.3 Comparison Between Materials

Tape DMP showed higher modulus values due to increased fiber content, while strength values were influenced by impregnation completeness. Commercial tapes demonstrated more balanced performance, while towpregs required more extensive consolidation to achieve comparable results.

5. Discussion

The results demonstrate that thermoplastic impregnation is not completed during the hot-melt stage. Instead, it progresses through multiple stages, each contributing to final material quality.

5.1 Multi-Stage Impregnation Mechanism

• Hot-melt impregnation — initial resin distribution and partial wetting
• Pultrusion — partial consolidation under limited time and pressure
• Compression moulding — final impregnation and void elimination

This staged process explains why even well-designed prepregs require additional consolidation.

5.2 Process Limitations

The main limitations of hot-melt impregnation include:

• Insufficient pressure for deep fiber penetration
• Limited residence time
• High melt viscosity restricting flow

Pultrusion improves consolidation but is constrained by short processing times. Compression moulding provides the necessary conditions for complete impregnation due to higher pressure and longer dwell times.

5.3 Engineering Implications

The study highlights a fundamental contradiction in thermoplastic processing:

• Low viscosity improves impregnation
• High viscosity is needed for pressure build-up

This requires process parameters to be optimized differently at each stage rather than globally.

6. Conclusion

This study confirms that melt-impregnated thermoplastic tapes are effective intermediate materials but do not achieve complete impregnation independently. Instead, thermoplastic composite manufacturing must be understood as a multi-stage process involving progressive consolidation.

Key conclusions include:

• Hot-melt impregnation improves initial fiber wetting but remains incomplete
• Pultrusion provides partial consolidation but cannot eliminate all voids
• Compression moulding is essential for achieving near-complete impregnation
• Mechanical properties are strongly influenced by impregnation quality, not just fiber content
• Process design must consider the interaction between viscosity, pressure, and time across all stages

Ultimately, successful thermoplastic composite production depends on integrating all processing steps into a unified system rather than optimizing them individually.

References

Esfandiari, P., et al. (2022). Production and processing of pre-impregnated thermoplastic tapes by pultrusion and compression moulding. Journal of Composite Materials, 56(11), 1667–1676.

ASTM D1238 – Standard Test Method for Melt Flow Rates of Thermoplastics.

Gibson, R. F. (2016). Principles of Composite Material Mechanics.

Mallick, P. K. (2007). Fiber-Reinforced Composites: Materials, Manufacturing, and Design.

Strong, A. B. (2008). Fundamentals of Composites Manufacturing.